1. Field of the Invention
This invention relates to an instrument for measuring lengths, particularly to a sealed instrument for measuring lengths, comprising a carrier, a rule mounted on said carrier and provided with a scale, and an optoelectronic scanning unit which is movable along said scale and comprises a scanning plate, light emitters and light receivers, also comprising an actuating member, which constitutes a feeler and is operable to move said scanning unit along said scale and is connected to a part of the scanning unit by means which are yieldable transversely to the longitudinal direction of the scale, and auxiliary tracks for guiding a part of the scanning unit along the scale.
2. Description of the Prior Art
Such instruments are known from Laid-open German Applications Nos. 23 49 944 and 25 10 219. The measurement is to be taken in the longitudinal direction of the scale so that errors which would be due to a transverse deviation of the feeler, which may be coupled, e.g., to a spindle of a machine tool, will be precluded and a disengagement of the scanning plate from the rule or a change of the distance between the scanning plate and the rule will also be precluded. In the known instruments for measuring lengths the scanning plate, the light emitters and the light receivers are assembled in the scanning unit to form a rigid unit, which in most cases is accommodated in a separate housing, and the feeler is connected to that unit by means which are yieldable transversely to the longitudinal direction of the scale. Said connecting means comprises pressure-applying springs, which urge sliders or rollers carried by the scanning unit against the auxiliary tracks, which in most cases consist of two guiding surfaces which are at right angles to each other. If one of said auxiliary tracks is constituted by the rule, the sliders or rollers must be provided on the scanning unit at a relatively large distance from the scanning plate. For this reason an exact guidance of the entire scanning unit rather than only of the scanning plate relative to the scale is ensured and the rule must have a relatively large width so that it can be formed with proper guiding surfaces. The scanning unit, which is relatively movably supported on the feeler, has a relatively heavy weight so that relatively strong springs are required to ensure that the sliders or rollers of the scanning unit will reliably remain in engagement with the auxiliary tracks even when vibrations or shocks occur, which are inevitable when the instrument for measuring lengths is mounted on a machine tool. As a result, strong pressure forces are exerted on said auxiliary tracks. Another important disadvantage of the known measuring instruments of the present kind resides in that the leads connected to the light emitters and to the light receivers must also be laid so as to permit transverse movements of the scanning unit relative to the coupling member. Said light receivers are often provided on printed circuit boards, which are also provided with amplifiers or with other signal-processing means.
In order to accomplish other objects, namely, to permit a correction of a measurement, e.g., for a compensation of pitch errors of the scale, by a movement of the scanning unit relative to the actuating member, it is known to mount the scanning unit so that it can be adjusted relative to the actuating member in the longitudinal direction of the scale or can be tilted relative thereto and to provide a correcting track, which in most cases constitutes a correcting template, which has been designed as a result of a control measurement performed with a higher-ranking measuring system and is scanned by means of a feeler by which the scanning unit is correspondingly advanced or retracted or pivotally moved. Such designs are known from Laid-open German Applications Nos. 32 08 591 and 32 10 962. Laid-open German Application No. 32 10 962 discloses an embodiment which provides for a pivotal movement and in which only the scanning plate is pivotally moved whereas the remaining parts of the scanning unit are guides along the scale regardless of the pivotal movement. If it is desired in that case that the scanning unit should move along the scale, the entire scanning unit must be guided in the manner described first hereinbefore and must be connected to the feeler in the manner described hereinbefore. A tilting adjustment of the scanning plate will only be possible if that scanning plate is spaced a relatively large distance from the rule. That distance will change during a tilting movement at least adjacent to certain gratings so that signal distortion will occur and must be compensated. If a measuring movement along the scale is to be ensured, the overall structure will be even more expensive than the measuring instrument of the kind described first hereinbefore and all disadvantages of the latter instruments will be encountered. The same disadvantages will be encountered in the use of the structure disclosed in Laid-open German Application No. 32 08 591. In that embodiment the scanning plate is connected to springs, which constitute a parallel crank linkage and constrain the scanning plate to move parallel to itself relative to the remaining parts of the scanning unit. The adjustment of the scanning plate is again effected by a feeler, which scans a template. Alternatively, the adjustment may be effected by piezoelectric means under the control of suitable signals. It might be contemplated to guide the scanning unit in such a manner that it will necessarily move along the scale--although that concept has not been disclosed. But even in that case the longitudinal adjustment of the scanning plate relative to the scale may change the distance from that plate to the scale so that a larger distance from the scanning plate to the rule must be maintained.